Chemically-induced hepatobiliary dysfunction not associated with parenchymal necrosis and fatty infiltration of the liver occurs in two distinct forms. In drug induced intrahepatic cholestasis decreased biliary excretion of organic solutes is readily attributable to diminished bile flow. However, certain organochlorine pesticides (i.e., mirex and chlordecone) decrease biliary excetion of readily excretable organic anions (vis, polar metabolites of imipramine and phenolphthalein glucuronide) in the face of enhanced bile flow. Limited understanding of the processes underlying bile secretion per se and biliary excretion in particular impedes advance of mechanistic explanations for such toxic events. This proposal is aimed at determining if an active transport system for organic anions is operational at the site of the bile canaliculus and whether disruption of performance of said system underlies certain forms of liver injury. The tentative model for the active transport system involves establishment of a proton gradient by a H+ ATPase with subsequent symport of organic anions via a coupled porter system manipulations intended to alter biomembrane structure and organization (i.e., mirex or chlordecone treatment and temperature variation) will be employed to modulate performance of the proposed transport system. Two animal models, the laboratory rat in pesticide studies and rainbow trout in temperature studies will be used in limited in vivo and in vitro experiments. Application of bile canaliculi enriched fractions for vesicular transport assays will be the principal vehicle for characterization of the proposed transport system. Bioenergetic and kinetic features of taurocholate and phenolphthalein glucuronide uptake and efflux will be determined in membrane vesicles and related to criteria for active, carrier mediated transport. The influence of tempeature and toxicants on in vivo biliary excretion of organic anions will be assessed in terms of how such treatment influence in vitro performance of components of the proposed transport system (viz, H+ATPase and the coupled porter system). Mechanistic explanation for operation of the hepatobiliary system and how alterations of biomembrane structure and organization impact on its performance will be sought.